Method and apparatus to reject electrical interference in a capacitive liquid level sensor system

ABSTRACT

Methods, systems, and computer program products for measuring a capacitance between a probe and a liquid, pausing movement of the probe based on a rate of change of the capacitance, further measuring the capacitance while the probe is paused, and, based on the further measurements, performing one or more of: resuming movement of the probe, determining a position of the probe, aspirating liquid into the probe, and dispensing from the probe. Resuming movement of the probe can include returning iteratively to measuring a capacitance, and the further measuring can be performed for a time interval that can vary based on the further measured capacitance(s), a probe movement characteristic, and/or a sampling rate.

PRIORITY CLAIM

The present application is a divisional of U.S. application for patentSer. No. 10/975,658 filed Oct. 28, 2004, now U.S. Pat. No. 7,191,647,which claims priority from U.S. Provisional Application for Patent Ser.No. 60/481,581, filed Oct. 30, 2003, and entitled “Method and Apparatusto Reject Electrical Interference in a Capacitive Liquid Level SensorSystem” by Richard R. Harazin and Ronald A. Zweifel, the disclosures ofwhich are hereby incorporated by reference in their entireties.

BACKGROUND

1. Technical Field

The present application relates to liquid level sensor systems. Morespecifically, the present application relates to the rejection ofinterference which adversely affects the operation of a capacitiveliquid level sensor (LLS) system.

2. Description of Related Art

Withdrawing and dispensing precise volumes of a liquid withoutcontaminating the liquid is an important part of many clinicalapplications and laboratory tests. While attempts to address these needsmanually have failed to provide the needed accuracy and purity, a numberof automated or semi-automated liquid-measuring systems are presentlybeing used to gauge more precisely the small liquid volumes that need tobe withdrawn and/or dispensed.

One of the more popular types of liquid-measuring systems uses amotor-controlled pipette-like probe to aspirate or dispense a desiredamount of fluid from or into a container. The probe is movably mountedover the container and, using a precision-controlled motor, isvertically (z-axis) lowered into the container until the tip of theprobe reaches a desired level below or above the upper surface of theliquid (the meniscus). A desired amount of liquid is then withdrawn fromor dispensed into the container. Such systems have been designed to:minimize/reduce cross-contamination between the contents of differentcontainers, avoid splashing of the liquid during the aspiration (ordeposition) process, and minimize/reduce the portion of the probe thatmust be washed.

In many instances, the automated or semi-automated liquid-measuringsystem does not know beforehand the level of fluid contained within thecontainer. Nonetheless, the pipette-like probe must be lowered to acertain position with the expectation that the probe has been preciselypositioned with respect to the fluid level. Several systems control theposition of the probe tip without previously knowing the upper level ofthe liquid in the container by sensing for the upper level of the liquidin the container as the probe is being lowered. For example, ameasurement can be made of some electrical phenomena associated with achange in the capacitance between the probe and the liquid in thecontainer as the tip of the probe approaches the liquid. Thismeasurement may identify a liquid sense event (for example, penetratingthe meniscus or withdrawing from the meniscus) when the capacitancebetween the probe and the liquid reflects a change in voltage level thatis greater than a threshold reference level.

A well known system and technique for capacitive-based sensing of liquidlevel in a z-axis controlled liquid-measuring system is taught by U.S.Pat. No. 5,365,783, to Ronald A. Zweifel (the “Zweifel system”), thedisclosure of which is hereby incorporated by reference.

In some existing systems, the labware can collect and store staticelectricity. As the probe is moved closer and closer to the container,one or more static discharge events may occur between the probe andcontainer during probe movement. These static discharge events can causean instantaneous change in a signal indicative of the measuredcapacitance which can incorrectly be detected by the system as a liquidevent (for example, a false positive indication that the probe is in theliquid when in fact it is still positioned above the liquid). Second,the laboratories where these systems are commonly used are typicallyilluminated using fluorescent light fixtures. The electronic ballastsused by such fixtures emit high frequency electromagnetic radiation.Probes can thus act as an antenna with respect to such radiation, andthe corresponding noise signal from that antenna-captured radiation canadversely affect a signal indicative of the measured capacitance and cancause an incorrect detection of a liquid event.

SUMMARY

An embodiment of the present teachings processes a signal indicative ofa capacitance formed between a probe that is controlled to move intoand/or out of a container and a liquid contained within the container.The capacitance indicative signal can be filtered to produce a triggersignal indicative of an event with respect to probe movement. Thetrigger signal is also processed to detect occurrence of the event andmovement of the probe is paused in response to a detected eventoccurrence.

Another embodiment also processes a signal indicative of a capacitanceformed between a probe that is controlled to move into and/or out of acontainer and a liquid contained within the container. The capacitanceindicative signal can be low pass filtered to attenuate high frequencynoise and produce a filtered capacitance indicative signal. The filteredsignal can be high pass filtered to attenuate low frequency componentsand produce a trigger signal indicative of an event with respect toprobe movement. The trigger signal is then processed to detectoccurrence of the event and the filtered capacitance indicative signalis processed to determine whether the event is an interference event ora liquid sensing event.

In accordance with an embodiment, a liquid-measuring system includes aprobe that is controlled to move into and out of a container containinga liquid, where a capacitance forms between the probe and the liquid. Acapacitive sensing system coupled to the probe generates a signalindicative of the capacitance. A control system that receives thecapacitance indicative signal responds to an event-related changetherein by pausing the movement of the probe and further evaluating thecapacitance indicative signal while probe movement is paused todetermine whether the change was caused by an interference event or aliquid sensing event.

In one embodiment, a liquid-measuring system includes a probe that iscontrolled to move into and/or out of a container containing a liquid,where a capacitance can be detected that is indicative of a capacitancebetween the probe and the liquid. A capacitive sensing system coupled tothe probe generates a signal indicative of the capacitance. A controlsystem receives the capacitance indicative signal, filters thecapacitance indicative signal and processes the filtered capacitanceindicative signal to determine whether a change in the capacitanceindicative signal was caused by an interference event or a liquidsensing event.

The present teachings thus include a liquid-level sensing system havinga probe that is controlled to move into and/or out of contact with aliquid, a capacitive sensing system coupled to the probe to generate asignal indicative of a capacitance between the probe and the liquid, anda control system that receives the capacitance indicative signal, andbased thereon, pauses the movement of the probe and processes thecapacitance indicative signal while probe movement is paused todetermine whether the probe moved at least one of into and out ofcontact with the liquid. The control system can pause the movement ofthe probe based on a rate of change of a signal based on the capacitanceindicative signal. The control system can process the capacitiveindicative signal while the probe movement is paused to determine thatchanges in the capacitance indicative signal are based on signalinterference, such as, for example, high frequency noise, staticdischarge, etc. In an embodiment, the capacitance indicative signal is avoltage signal.

The control system can include a bandpass filter operable on thecapacitance indicative signal to generate a trigger signal, and movementof the probe is paused based on the trigger signal, for example, whenthe trigger signal exceeds a threshold.

The methods, systems, and computer program products presented hereininclude a liquid-measuring system that includes a probe that iscontrolled to move at least one of into and out of contact with aliquid, a capacitive sensing system coupled to the probe to generate asignal indicative of a capacitance between the probe and the liquid, anda control system that receives the capacitance indicative signal,filters the capacitance indicative signal, and processes the filteredcapacitance indicative signal to determine whether a change in thecapacitance indicative signal was caused by probe movement into and/orout of contact with the liquid.

In an embodiment, the present teachings include a system for processinga signal indicative of a capacitance formed between a probe that iscontrolled to move into and/or out of contact with a liquid, and theliquid, where the system includes a low pass filter that operates on thecapacitance indicative signal to produce a filtered capacitanceindicative signal, a high pass filter that operates on the filteredcapacitance indicative signal to produce a trigger signal indicative ofan event with respect to probe movement, and processor instructionsoperable on the filtered capacitance indicative signal and triggersignal, where the instructions detect occurrence of the event from thetrigger signal and the instructions determine, based on the filteredcapacitance signal, whether the event is a liquid sensing event. Theprocessor instructions can pause the movement of the probe in responseto a detected event occurrence, and can process the filtered capacitanceindicative signal while probe movement is paused to determine whetherthe detected event occurrence is an interference event or a liquidsensing event. The processor instructions can thus restart and/or resumemovement of the probe if the detected event occurrence is determined tobe an interference event.

The teachings include a method for processing a signal indicative of acapacitance formed between a probe that is controlled to move intoand/or out of contact with a liquid, and the liquid, including low passfiltering the capacitance indicative signal to produce a filteredcapacitance indicative signal, high pass filtering the filteredcapacitance indicative signal to produce a trigger signal indicative ofan event with respect to probe movement, processing the trigger signalto detect occurrence of the event, and processing the filteredcapacitance indicative signal to determine whether the event is a liquidsensing event. Also included is a restarting of movement of the probe ifthe detected event occurrence is determined to be an interference event.

Accordingly, in an embodiment, the teachings include a system forprocessing a signal indicative of a capacitance between a probe that iscontrolled to move into and/or out of contact with a liquid, and theliquid, the system including a bandpass filter operable on thecapacitance indicative signal to produce a trigger signal indicative ofan event with respect to probe movement, and a processor havinginstructions operable on the trigger signal to detect occurrence of theevent from processing of the trigger signal and to pause the movement ofthe probe in response to a detected event occurrence.

In an embodiment, the teachings include a method for processing a signalindicative of a capacitance formed between a probe that is controlled tomove into and out of contact with a liquid, and the liquid, includingfiltering the capacitance indicative signal to produce a trigger signalindicative of an event with respect to probe movement; processing thetrigger signal to detect occurrence of the event; and pausing themovement of the probe in response to a detected event occurrence. Thefiltering can include high pass filtering and/or low pass filtering.Processing the trigger signal includes detecting when a trigger signalexceeds a threshold. Pausing the movement of the probe includesprocessing the filtered capacitance indicative signal after pausing themovement, and, based on the processing, determining whether the event isassociated with movement of the probe into contact with the liquid,and/or movement of the probe out of contact with the liquid. Thedetermining also includes determining that the event is not associatedwith movement of the probe into contact with the liquid and/or movementof the probe out of contact with the liquid, and resuming movement ofthe probe. Accordingly, the determining can include determining that theevent is associated with movement of the probe into contact with theliquid and/or movement of the probe out of contact with the liquid, anddetermining a level of the fluid based on the position of the probeand/or a sensor associated with the probe.

In one embodiment, the teachings include measuring a capacitance betweena probe and a liquid, pausing movement of the probe based on a rate ofchange in the capacitance, further measuring the capacitance while theprobe is paused, and, based on the further measurements, performing oneor more of: resuming movement of the probe, determining a position ofthe probe, aspirating liquid into the probe, and, dispensing from theprobe. Resuming movement of the probe includes a method that includesreturning iteratively to measuring a capacitance. The further measuringcan be performed for a time interval that can be based on predeterminedtime interval, and/or based on the further measured capacitance. Infurther measuring, the method can include comparing data based on thefurther measurements to at least one threshold, changing a counter valuebased on the comparisons, and, determining the time interval based onthe changed counter value. The further measuring can be based on asampling interval and/or a probe movement characteristic that caninclude a probe speed, a probe velocity, and a probe acceleration, forexample.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present teachings may be acquiredby reference to the following Detailed Description when taken inconjunction with the accompanying Drawings wherein:

FIG. 1 is a block diagram of a z-axis controlled automatedliquid-measuring system;

FIG. 2 is a graph illustrating over time a capacitive sensing systemoutput signal as a probe moves towards and then contacts the meniscus ofa liquid in a container;

FIG. 3 is a block diagram of a z-axis controlled automatedliquid-measuring system in accordance with an embodiment of the presentteachings;

FIG. 4 is a block diagram of an interference rejection processing schemeused by a system according to FIG. 3;

FIG. 5 is a flow chart that diagrams operation of one “DEBOUNCE STATEMACHINE” as shown in FIG. 4;

FIG. 6 is a graph illustrating over time a capacitive sensing systemoutput signal, a low pass filtered signal and a bandpass (trigger)signal as a probe moves towards and then contacts the meniscus of aliquid in a container in presence of two static discharge events;

FIG. 7 is a graph showing the signals of FIG. 6 in more detailsurrounding one of the static discharge events; and

FIG. 8 is a graph showing the signals of FIG. 6 in more detailsurrounding the liquid level sensing event.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a z-axis controlled automated liquid-measuring system10 which provides control over the position of a probe 11 with respectto an unknown level of liquid in a conventional laboratory container 14.Movement of the probe 11 in the z-axis into or out of the container 14is controlled by a conventional servo-drive 12 which is controlled by amicrocomputer or CPU 13. The CPU 13 determines the position of the probe11 with respect to the level of the liquid in the container 14 bymonitoring a signal 15 representing capacitance between: the probe 11and the liquid in the container 14, the liquid in the container 14, andground. The CPU may also be programmed to control a liquid handler 16(e.g., syringe) which can aspirate or dispense a desired amount of fluidfrom or into the container 14 through the probe 11 via a flexible tube18 made from a chemically inert material such as Teflon, for example.

The stray capacitance, depicted as C₁, between the probe 11 and theliquid in the container 14 slowly increases as the probe 11 is movedtoward the liquid. When the probe contacts the liquid in the container14, the stray capacitance, C₁, changes virtually instantaneously. Acapacitive sensing system 100 is coupled, via a conductor 31 a to detectchanges in this stray capacitance, and provide corresponding sensor datato the CPU 13 in the form of signal 15 which is indicative of acapacitance between the probe 11, the liquid in the container 14, andground.

The stray capacitance C₁ is monitored by the system 100 through asampling operation that with each sample instant applies a voltage tothe probe 11/container 14 (i.e., to the capacitance C₁) and measures thevoltage charge on the capacitance C₁ to detect and latch a peak voltagedeveloped on the capacitance C₁. This latched peak voltage is output asthe signal 15 for application to the CPU 13. In embodiments, the signal15 is a digital signal representation of the, sampled peak voltagevalue. When the probe 11 contacts the liquid in the container 14, arapid change occurs in the stray capacitance which causes the measuredvoltage developed on the capacitance to decrease rapidly. This decreasedvoltage signal level is detectable in the output signal 15 and does notchange for as long as the probe 11 remains in the liquid (assuming noevaporation/loss of liquid).

As an example, at the start of each sampling cycle, the voltage at probe11 is initially held at 0 Volts via a closed switch (not shown) betweenground and conductor 31 a. Next, the switch is opened so that theillustrated voltage (e.g. 24V) is applied through an impedance 17 to thecapacitance C₁. The impedance 17 and the capacitance C₁ form an RC timeconstant such that the voltage at the probe 11 increases at a rateproportional to the capacitance C₁. After a fixed time (e.g.,approximately 10 microseconds), the peak voltage at probe 11 is measuredby the system 100 via conductor 31 a. This voltage measurement isindicative of capacitance C₁ and is sent to the computer 13 via signal15. The switch is then closed, thus discharging the capacitance C₁ andresetting the voltage at probe 11 to 0 Volts. The foregoing cycle can becontinuously repeated at a fixed frequency.

Conductor 31 a can be implemented as the inner conductor of a coaxialcable. Conductor 31 b can be the shield of the coaxial cable which canbe electronically driven at the same voltage as conductor 31 a (thusproviding near 0 capacitance between the conductor 31 a and shield 31 b)to reduce changes in cable capacitance (e.g. when the cable is moved orcomes in contact with an object), and to reduce external electronicnoise.

The capacitive sensing system 100 may be configured in a number of waysto generate the output signal 15 representing the peak voltage charge atthe capacitance C₁. One exemplary circuit for implementing thecapacitive sensing system is taught by U.S. Pat. No. 5,365,783, toRonald A. Zweifel, the disclosure of which is hereby incorporated byreference.

As the probe is moved toward (or away) from the liquid, the capacitanceC₁ changes and thus the peak voltage charge storable on the capacitanceC₁ at each sampling instant also changes. The storable peak voltagecharge (provided in the signal 15) is monitored and when it changes(e.g., decreases) at a rate exceeding a rate expected for probemovement, provides an indication of the occurrence of an event such as aliquid level sense event when the probe 11 penetrates the meniscus ofthe fluid within the container. However, it is known thatelectromagnetic interference can cause momentary increases/decreases inthe storable peak voltage charge where no meniscus penetration hasoccurred. In such situations, the rate of voltage change (e.g.,decrease) which is detected by the capacitive sensing system could beprocessed by the computer 13 and incorrectly identified as a liquidlevel sense event (i.e., a false positive indication).

Reference is now additionally made to FIG. 2 wherein there is shown agraph illustrating over time the capacitive sensing system 100 outputsignal 15 as the probe moves towards and then contacts the meniscus of aliquid in the container 14. The output signal 15 is a digital signalrepresentation of the sampled peak voltage value developed on thecapacitance C₁. The y-axis of the graph accordingly measures thedigitized sampled peak voltage value. The x-axis measures time in termsof a number of samples, wherein the time between sampling instantscould, for example, be 500 μsec. The graph shows the output signal 15peak voltage value generally rising with time as the probe 11 is movedby the servo-drive 12 into the container 14 closer and closer to themeniscus of the contained fluid. When the probe 11 touches the meniscusand penetrates into the fluid contained within the container 14 (atabout the sampling instance identified by reference 50), the measuredpeak voltage at the capacitance C₁ decreases virtually instantaneouslyas evidenced by the voltage drop (voltage step) in output signal 15. Thecomputer 13 detects this rate of change in voltage in the receivedoutput signal 15 and provides a control signal to the servo drive 12 tosuspend movement of the probe 11 in the z-axis since the probe haspenetrated the meniscus of the fluid contained within the container. Thecomputer 13 may further provide a control to the liquid handler 16 toaspirate or dispense a desired amount of fluid from or into thecontainer 14 through the probe 11.

FIG. 2 further shows an output signal 15′ that has been perturbed (see,at about the sampling instance identified by reference 52) byinterference. For example, the interference-related perturbation couldbe caused by a static discharge event occurring between the probe andcontainer (or some other dynamic interference event). Theinterference-related perturbation could additionally and/or optionallybe caused by probe 11 receipt of high frequency electromagneticradiation (e.g., emitted by fluorescent lighting fixtures) that isimpressed upon the peak voltage measurement (or some other steady stateinterference event). In either situation, the interference can cause themeasured peak voltage at the capacitance C₁ to decrease and thus producea voltage drop in output signal 15′. In some instances, this voltagedrop possesses, at least over a certain number of sampling instants, thecharacteristics of a voltage step. The computer 13 accordingly coulddetect this step-like voltage drop in the received output signal 15′,and then incorrectly interpret it as an indication that the probe 11 hasreached the meniscus. Responsive thereto, the computer would incorrectlyprovide control/instructions to the servo drive 12 to terminate furthermovement of the probe 11 in the z-axis. This false positive meniscusdetection, however, can result in the probe 11 being incorrectlypositioned in the z-axis within the container 14 with respect to thecontained liquid. At this position, a proper fluid aspiration ordeposition process using the liquid handler 16 cannot be accomplished.

Reference is now made to FIG. 3 wherein there is shown a block diagramof a z-axis controlled automated liquid-measuring system 10′, inaccordance with an embodiment of the present teachings, for controllinga single probe with respect to liquid in a laboratory container. Likereference numbers in FIG. 3 refer to like or similar parts in FIG. 1,the description of which will not be repeated.

The system 10′ of FIG. 3 differs from the system 10 of FIG. 1 in thatthe computer 13 implements an interference rejection instructions 60 toaddress dynamic (e.g., static discharge) and steady state (e.g.,electromagnetic radiation) interference events. A block diagram of theinterference rejection instructions 60 are illustrated in FIG. 4.

A digital low pass filter (LPF) 70 processes the digitized sampled peakvoltage value developed on the capacitance C₁ to provide low passfiltered data 72. The LPF 70 can be implemented as a series of twodigital low pass filters for the reasons discussed herein. The operationof the LPF 70 effectively reduces periodic electromagnetic interference(EMI) which may be present in the received peak voltage data signal 15.Thus, the LPF 70 attenuates high frequency signal components from thesignal 15 such as the steady state noise which may be captured by theprobe 11 from external radiation sources such as fluorescent lightfixtures.

The two series-connected digital low pass filters are based on abilateral Z transform. The formula used for each of LPF1 and LPF2 is asfollows:LPF _(n) =LPF _(n−1)+((NEW DATA_(n) −LPF _(n−1))/TAU)where:

LPF_(n) is the current low pass filter output value;

LPF_(n−1) is the previous low pass filter output value;

NEW DATA_(n) is the current input value (signal 15); and

TAU is the time constant (as TAU gets larger, F3 db (the cutofffrequency) gets lower).

By routing the output of one filter (LPF1) as an input to a secondfilter (LPF2), a 2 pole LPF 70 can be formed to increase the cutoffslope (e.g., 12 db per octave instead of 6 db per octave) to providebetter noise rejection above F3 db. TAU for each filter stage may bechosen in view of the probe movement speed (e.g., 100 mm/sec), a samplerate, and a specified penetration distance prior to stopping (e.g., nomore than 1 mm penetration into the sample). Empirical methods can beused to determine TAU, where generally, in the illustrated embodiments,larger-valued TAU correspond to delayed response. As a example, LPF1 mayhave TAU=4 and provide a f3 db of 100 Hz, while LPF2 may have TAU=8 andprovide a F3 db of 40 Hz.

The low pass filtered output data 72 is applied to a “debounce” statemachine 74, and also into a digital high pass filter (HPF) 76. Thecombination of the LPF 70 and HPF 76 creates a bandpass filter. Bandpassfiltering of the received peak voltage data signal 15 generates a“trigger” signal 80 that is amplified by amplifier 78 and applied to thedebounce state machine 74.

The digital HPF 76 is created using subtraction, the formula being:HPF _(n) =LPF _(n) −LPF _(n−3)where:

HPF_(n) is the high pass filter output;

LPF_(n) is the current output value from the second low pass filter; and

LPF_(n−3) is the 3rd previous output value from the second low passfilter.

As an example, the f3 db (cutoff frequency) for the HPF 76 is set atabout 80 Hz.

The net effect of running the LPF output data 72 into the HPF is tocreate an overall bandpass filter effect with a center frequency ofabout 90 Hz. The amplifier 78 applies a gain of 4 to the HPF output data(the “trigger” signal 80). The gain value, however, is user-selectableand can be adjusted as for compatibility with debounce state machine 74operations such as threshold comparing of the trigger signal 80 in orderto detect that an event has occurred.

Upon detection of a trigger 80 signifying that an event has occurred,the debounce state machine 74 generates a pause control signal 82 whichis sent to a motion control process 84 within the computer. Responsivethereto, the motion control process 84 signals the servo drive 12 tomomentarily (at least) pause probe 11 movement. With the probe 11movement paused, the debounce state machine 74 then analyzes a largeportion (i.e., multiple sample points) of subsequent LPF output data 72to determine whether a voltage step in the signal 15 has occurred (see,FIG. 2, reference 50) that is indicative of a liquid event such aspenetration of the liquid meniscus within the container, or if instead aperturbation in the signal 15 has occurred (see, FIG. 2, reference 52)that is indicative of interference or noise where liquid penetrationposition has not yet been reached. For example, in the situation wherethe event is a static discharge event, pausing of probe 11 movement isbeneficial as it allows for the static interference to dissipate. If theevent is determined to be a liquid event (because the subsequentmultiple sample points of the LPF output data 72 show a voltage step),then the debounce state machine 74 sets a liquid level sense (LLS) flag86 to true. The setting of this flag indicates to the computer 13 thatthe probe 11 has penetrated the meniscus of the liquid within thecontainer 14. Responsive thereto, the computer 13 may signal the liquidhandler 16 to aspirate or dispense a desired amount of fluid from orinto the container 14 through the probe 11. If, on the other hand, theevent is determined to be noise (because the subsequent multiple samplepoints of the LPF output data 72 fail to show a voltage step), the eventis rejected by the debounce state machine 74 and a “ramp up” command 88is issued. Responsive thereto, the motion control process 84 signals theservo drive 12 to continue probe 11 movement in the z-axis (and/or someother axis) in the search for liquid. If the analysis of the multiplesample points of the LPF output data 72 by the debounce state machine 74is inconclusive with respect to identifying the event (e.g., too muchnoise to make a decision), and thus probe movement has been paused fortoo long a period of time, the debounce state machine 74 reports a“liquid not found” error 90.

Reference is now made to FIG. 5 wherein there is shown a flow diagramillustrating logical operations performed by one embodiment of adebounce state machine 74. As discussed above, one purpose of thismachine is to determine whether an event is a liquid sense event, oranother event such as a noise/interference event. The process fordiscriminating between a liquid sense event and a noise/interferenceevent will now be explained.

The trigger signal 80 is compared to a user defined liquid level sense(LLS) threshold (200 and 202). If the signal 80 exceeds the LLSthreshold, then an “event” is said to have occurred (path 204). If thesignal does not exceed the LLS threshold, the process of 200 and 202 isrepeated for another sample point (path 206). As an example, thiscomparison operation can determine whether the signal 80 drops below acertain value point (the threshold), or alternatively whether amagnitude of change in the signal 80 over a given number of samplesexceeds the threshold. Other comparisons for event determination basedon signal 80 value could also be used.

If the test for an event has been satisfied, in 208 the LPF output data72 for the sample point immediately preceding the detected event issaved, and a pause control signal 82 is generated and sent to the motioncontrol process 84 to pause probe 11 movement. For example, the z-axismotor in the servo drive 12 may be paused by setting the motor state to“paused”.

At 210, the current sample point of the LPF output data 72 is comparedto the saved LPF output data 72 sample point (from 208). If thedifference between the current LPF output data 72 sample and the savedLPF output data 72 sample point (from 208) exceeds the LLS threshold(path 212), a pass counter is incremented 214. Otherwise (path 216), thepass counter is decremented 218. In either case, a trial (time) counteris incremented 220. This trial (time) counter is then compared 222 witha time limit. If the time limit (e.g., one second) has not been exceeded(path 224), a next current LPF output data 72 sample is obtained 226 andthe process returns to 210 to compare that current LPF output data 72sample to the saved LPF output data 72 sample point (from 208). In thisway, the process iterates, while the probe movement is paused, andevaluates subsequent new samples of signal 72 as the process attempts todiscern whether the detected event is a liquid event or, for example,noise/interference.

Although the process of FIG. 5 shows the use of the same LLS thresholdvalue for both the event test of 200 and 202 and the voltage test of210, it will be understood that these thresholds are user definable andmay, if desired, be set to different values. Altering of the thresholdvalues affects the sensitivity of the process to detect events andvoltage indicative of a liquid sense event. As an example, in anembodiment with a sampling rate of 2 KHz, a user may set the passthreshold to one-hundred, and the fail threshold to negativeone-hundred, although such example is provided for illustration and notlimitation.

To determine whether the event is a liquid event, the path 212 includesa test 228 where the pass counter is compared to a user defined PASSthreshold. If, after looping through/iterating and evaluating subsequentsamples, the pass counter has not equaled and/or surpassed the PASSthreshold, the loop process continues with the incrementing of the passcounter 214, the testing of the trial (time) counter 220 and 222, andthe acquisition of the next current LPF output data 72 sample 226.However, if the pass counter has equaled and/or surpassed the PASSthreshold (path 232), the scheme concludes that it has found a liquidevent and in 234, the debounce state machine 74 sets the liquid levelsense (LLS) flag 86 to true, indicating to the computer 13 that theprobe 11 has penetrated the meniscus of the liquid within the container14. At 234, the trial (time) counter is further reset.

To determine whether the event is a noise/interference event, the path216 includes a test 236 where the pass counter is compared to a userdefined FAIL threshold. If, again after looping through/iterating andevaluating subsequent samples, the pass counter has not reached the FAILthreshold, the loop process continues with the decrementing of the passcounter 218, the testing of the trial (time) counter 220 and 222, andthe acquisition of the next current LPF output data 72 sample 226.However, if the pass counter has equaled and/or surpassed the FAILthreshold (path 240), the process concludes it has found a noise eventand in 242, the debounce state machine 74 rejects the event and issues a“ramp up” command 88 ordering the computer 13 to signal the servo drive12 to resume/continue probe 11 movement in the z-axis in the search forliquid.

If the testing of the trial (time) counter in 222 indicates that thetime limit has been passed (path 244), this indicates that the analysisof the multiple sample points of the LPF output data 72 by the debouncestate machine 74 is inconclusive with respect to identifying the event(perhaps because there is too much noise in the signal to make adecision). In 246, the debounce state machine 74 concludes that probemovement has been paused for too long a period of time without beingable to come to an event-type conclusion, and then reports the “liquidnot found” error 90. In 248, the trial (time) counter is reset.

It will be recognized by those skilled in the art that the processperformed within the box 250 (comprising 212-220, 228-230 and 236-238)in essence integrates the PASS/FAIL threshold data with respect to thecomparison of the plurality of loop collected current LPF output data 72samples to the saved LPF output data 72 sample point (from 208) in orderto reach a decision as to whether the event is a liquid event ornoise/interference. The integration operation helps reduce (i.e.,debounce) noise that may occur while the data is being analyzed.

Reference is now made to FIG. 6 wherein there is shown a graphillustrating over time a capacitive sensing system output signal 15, alow pass filtered signal 72 and a bandpass (trigger) signal 80(generated by the system of FIGS. 3 and 4) as a probe 11 moves towardsand then contacts the meniscus of a liquid in a container 14 in presenceof two static discharge events. The output signal 15 is a digital signalrepresentation of the sampled peak voltage value developed on thecapacitance C₁. The low pass filtered signal 72 is a digital signalrepresentation of the low pass filtered output signal 15. The left-handy-axis of the graph accordingly measures the digitized sampled peakvoltage value. The bandpass filtered (trigger) signal 80 is a digitalsignal representation of the bandpass filtered output signal 15. Sincethe bandpass filtering operation removes DC offset present in the signal15, the left-hand y-axis cannot be properly scaled in the illustrationto show the signal 80 with the signals 15 and 72 in connection with thesame y-axis. Accordingly, the right-hand y-axis is used to measure thedigitized sampled and bandpass filtered peak voltage value for signal80. The x-axis measures time in number of samples, wherein the timebetween sampling instants could, for example, be 500 μsec. Theillustrated time domain is applicable to all three signals 15, 72 and80.

The graph shows the output signal 15 peak voltage value generally risingwith time as the probe 11 is moved by the servo-drive 12 into thecontainer 14 closer to the meniscus of the contained fluid. The low passfiltered output signal 72 closely tracks the signal 15 (again, withattenuated high frequency interference effects), but does not includeinterference components which could distort or perturb the signal 15 andperhaps cause a false positive event detection. At the sampling instancereferenced 90 a first static discharge event occurs (see, also, FIG. 7).This injected dynamic interference from the static discharge perturbsthe signals 15 and 72 by causing the measured peak voltage at thecapacitance C₁ to decrease and thus produce a voltage drop. The bandpassfiltered trigger signal 80 spikes 92 as a result of the perturbation inthe signals 15 and 72. If the magnitude of this spike exceeds the LLSthreshold (FIG. 5, 200 and 202), then the debounce state machine 74 ofthe interference rejection scheme 60 detects an event 208. Otherwise,the perturbation is ignored. In this instance, the spike in signal 80has surpassed the LLS threshold (in the negative direction) thusindicating that an event has occurred.

It is unknown at this point in time by the system 10′ whether this eventis noise (interference) or a liquid sense event. To make thisdetermination, the system 10′ first causes a pause to z-axis movement ofthe probe 11, and then the integration process 250 of FIG. 5 is executedto evaluate a number of subsequent sampled values. In effect, thisintegration process 250 compares value of the low pass filtered signal72 to a pre-event value over a plurality of sampling points while theprobe 11 is stationary. In one embodiment, if a number of sampling pointvalues are found to be below the pre-event value by at least the LLSthreshold such that a PASS threshold (214 and 228) is satisfied, theprocessing scheme considers the event a liquid level sense event.Conversely, if a number of subsequent sampling point values are foundwithin the LLS threshold of the pre-event value such that the FAILthreshold (218 and 236) is satisfied, the processing scheme considersthe event a noise/interference event. For the example illustrated inFIGS. 6 and 7, the samples of the signal 72 following the event may dropinitially below the pre-event value but thereafter return to and remainstable at approximately the pre-event value. Thus, this event, as isappropriate since it was caused by a static discharge, would be found bythe processing scheme to be noise since the loop through path 216 wouldbe taken iteratively to decrement the pass counter and the FAILthreshold comparison would eventually be satisfied. The ramp-up signal88 would then be issued 242 and probe 11 movement would resume.

A subsequent second static discharge event at reference 94 would behandled by the system 10′ in an identical manner as discussed above.

Following continuation of probe z-axis movement towards the liquid, thegraph shows the output signal 15 peak voltage value generally risingwith time as the probe 11 is moved by the servo-drive 12 into thecontainer 14 and closer to the meniscus of the contained fluid. The lowpass filtered output signal 72 tracks the signal 15. At the samplinginstance referenced 96, the probe 11 moves towards and then contacts themeniscus of the liquid in the container 14 (see, also, FIG. 8). Thepenetration of the liquid meniscus causes a drop in the measured peakvoltage at the capacitance C₁. The bandpass filtered signal 80 spikes 98as a result of the voltage drop in the signals 15 and 72. If themagnitude of this spike exceeds the LLS threshold (FIG. 5, 200 and 202),then the debounce state machine 74 of the interference rejectionprocessing scheme 60 detects an event 208. Otherwise, the voltage dropis ignored. In this instance, the spike in signal 80 is below the LLSthreshold thus, indicating that an event has occurred.

Again, it is unknown at this point in time by the system 10′ whetherthis event is noise (interference) or a liquid sense event. To make thisdetermination, the system 10′ pauses further z-axis movement of theprobe 11 and the integration process 250 of FIG. 5 is executed toevaluate a number of subsequent sampled values. In effect, thisintegration process 250 compares the value of the low pass filteredsignal 72 to the pre-event value over a plurality of sampling pointswhile the probe 11 is stationary. If a number of sampling point valuesare found to be below the pre-event value by at least the LLS thresholdsuch that the PASS threshold 214 and 228 is satisfied, the processingscheme considers the event a liquid level sense event. Conversely, if anumber of subsequent sampling point values are found within the LLSthreshold of the pre-event value such that the FAIL threshold 218 and236 is satisfied, the processing scheme considers the event anoise/interference event. For the example illustrated in FIGS. 6 and 8,the subsequent samples of the signal 72 following the event drop belowand remain below the pre-event value. Thus, this event, as isappropriate since it was caused by probe 11 penetration into themeniscus, would be found by the processing scheme to be a liquid senseevent since the loop through path 212 would be taken iteratively toincrement the pass counter and the PASS threshold comparison wouldeventually be satisfied.

It will accordingly be recognized that operation of the system 10′presented herein reduces the number of LLS false positives (i.e.,erroneously sensing liquid before contact is made) and hence increasesthe reliability of system operation.

The illustrated system thus provides a “trigger” signal that indicatesan event and allows for a control to stop probe movement to reducecontamination, a low pass filtered data signal that reduces theintensity of periodic, high frequency EMI, a pausing probe movementwhich allows a longer LPF time constant (i.e., stronger filtering,improved noise rejection) and for a large portion of the LPF data signalto be analyzed. Pausing probe movement can also allow the tip speedduring normal (i.e., noise free) operation to remain relatively high,and can also distribute events in time for event discrimination (e.g.,discriminate between closely timed static event and liquid events).Pausing probe movement also allows static to be dissipated so that asthe tip continues to search for liquid, less static is encountered.Accordingly, probe movement pausing is allowed to occur multiple timesduring a liquid level search, so multiple layers of static charge can bediscriminated. In cases where a droplet exists on the probe end, theliquid level may still be found accurately since probe movement ispaused during the initial level sense, and the liquid surface may wickthe droplet away, creating a noise event such that the probe willcontinue to search for the actual liquid level.

What has thus been described, in one embodiment, are methods and systemsin which periodic electromagnetic interference (EMI), pulsed EMI, andstatic electrical interference are reduced and/or rejected in acapacitive liquid level sensor (LLS) system. Digital filters can reduceperiodic EMI and pause probe (and/or sensor) movement when a liquidsense event or noise event occurs. While the probe is paused, themethods and systems can process the filtered LLS data to determinewhether a step change in the LLS data has occurred. Such a step changein the LLS data signifies that the probe tip has contacted (or separatedfrom), or moved into and/or out of contact with the liquid. Becausenoise due to static or EMI is generally pulses of short duration anddoes not cause a step change in the LLS data, the noise is rejected andthe probe is commanded to continue its movement.

Although the primary implementation of the teachings is illustrated withrespect to the detection of probe penetration into a meniscus of a fluidwithin a container, it will be understood that the system and processare equally applicable to the detection of a probe leaving a meniscus.Thus, as used herein, the term “liquid sense” refers to the probe eitherentering or leaving the liquid within the container. Further, althoughthe illustrated embodiments indicate liquids that are associated with acontainer, the present methods and systems are not limited to liquids ina container and can be applied to liquids absent in a container, forexample, liquids that are present on a surface and held thereon bysurface tension.

Although the system 10′ is illustrated controlling the operation of asingle probe 11, it will be understood by those skilled in the art thesystem 10′, along with the processing scheme 60, is well suited for thecontrol of plural probes.

Although embodiments of the method and apparatus of the presentteachings have been illustrated in the accompanying Drawings anddescribed in the foregoing Detailed Description, it will be understoodthat the teachings are not limited to the embodiments disclosed, but arecapable of numerous rearrangements, modifications and substitutionswithout departing from the spirit of the teachings as set forth anddefined by the following claims.

1. A system for processing a signal indicative of a capacitance formedbetween a probe that is controlled to move at least one of into and outof contact with a liquid, and the liquid, the system comprising: a lowpass filter that operates on the capacitance indicative signal toproduce a filtered capacitance indicative signal; a high pass filterthat operates on the filtered capacitance indicative signal to produce atrigger signal indicative of an event with respect to probe movement;and processor instructions operable on the filtered capacitanceindicative signal and trigger signal, where the instructions detectoccurrence of the event from the trigger signal and pause the movementof the probe in response to a detected event occurrence, and wherein theprocessor instructions include instructions to process the filteredcapacitance indicative signal while the probe movement is paused todetermine whether the detected event occurrence is an interference eventor a liquid sensing event.
 2. The system of claim 1 wherein theprocessor instructions restart movement of the probe if the detectedevent occurrence is determined to be an interference event.
 3. Thesystem of claim 2 in which the processor instructions restart movementof the probe in a direction that is the same as a direction it wasmoving prior to pausing the probe movement.
 4. A method for processing asignal indicative of a capacitance formed between a probe that iscontrolled to move at least one of into and out of contact with aliquid, and the liquid, the method comprising: low pass filtering thecapacitance indicative signal to produce a filtered capacitanceindicative signal; high pass filtering the filtered capacitanceindicative signal to produce a trigger signal indicative of an eventwith respect to probe movement; processing the trigger signal to detectoccurrence of the event; pausing the movement of the probe in responseto a detected event occurrence; and processing the filtered capacitanceindicative signal while the probe movement is paused to determinewhether the detected event occurrence is an interference event or aliquid sensing event.
 5. The method of claim 4 further includingrestarting movement of the probe if the detected event occurrence isdetermined to be an interference event.
 6. The method of claim 5 inwhich the movement of the probe is restarted in a direction that is thesame as a direction it was moving prior to pausing the probe movement.7. A system for processing a signal indicative of a capacitance formedbetween a probe that is controlled to move at least one of into and outof contact with a liquid, and the liquid, the system comprising: a lowpass filter and a high pass filter, wherein one of the low pass filterand the high pass filter provides a filtered capacitance indicativesignal from the capacitive indicative signal, and together the low passfilter and the high pass filter provide a trigger signal indicative ofan event with respect to probe movement; and a processor comprisingprocessor instructions operable on the trigger signal, where theinstructions detect occurrence of the event from the trigger signal andpause the movement of the probe in response to a detected eventoccurrence, and wherein the processor instructions include instructionsto process the filtered capacitance indicative signal while the probemovement is paused to determine whether the detected event occurrence isan interference event or a liquid sensing event.
 8. The system of claim7 wherein the processor instructions restart movement of the probe ifthe detected event occurrence is determined to be an interference event.9. A method for processing a signal indicative of a capacitance formedbetween a probe that is controlled to move at least one of into and outof contact with a liquid, and the liquid, the method comprising: lowpass filtering and high pass filtering the capacitive indicative signal,wherein one of the low pass filter and the high pass filter provides afiltered capacitance indicative signal from the capacitive indicativesignal, and together the low pass filtering and the high pass filteringprovides a trigger signal indicative of an event with respect to probemovement; processing the trigger signal to detect occurrence of theevent; pausing the movement of the probe in response to a detected eventoccurrence; and processing the filtered capacitance indicative signalwhile the probe movement is paused to determine whether the detectedevent occurrence is an interference event or a liquid sensing event. 10.The method of claim 9 further including restarting movement of the probeif the detected event occurrence is determined to be an interferenceevent.
 11. The system of claim 10 in which the movement of the probe isrestarted in a direction that is the same as a direction it was movingprior to pausing the probe movement.
 12. A method for processing asignal indicative of a capacitance formed between a probe that iscontrolled to move at least one of into and out of contact with aliquid, and the liquid, the method comprising: low pass filtering thecapacitance indicative signal to produce a filtered capacitanceindicative signal; high pass filtering the filtered capacitanceindicative signal to produce a trigger signal indicative of an eventwith respect to probe movement; processing the trigger signal to detectoccurrence of the event; pausing the movement of the probe in responseto a detected event occurrence; processing the filtered capacitanceindicative signal while the probe movement is paused to determinewhether the detected event occurrence is an interference event or aliquid sensing event; restarting movement of the probe if the detectedevent occurrence is determined to be an interference event, wherein theprobe movement is restarted in a direction that is the same as adirection in which the probe was moving prior to pausing the probemovement; and not restarting movement of the probe if the detected eventoccurrence is determined to be a liquid sensing event.